Liquid crystal displays or flat panels are commonly used for active matrix displays such as computer and television monitors, personal digital assistants (PDAs), cell phones and the like. Generally, flat panels comprise two glass plates having a layer of liquid crystal material sandwiched therebetween. At least one of the glass plates includes at least one conductive film disposed thereon that is coupled to a power supply. Power supplied to the conductive material film from the power supply changes the orientation of the liquid crystal material, creating patterns such as text or graphics that may be seen on the display. One fabrication process frequently used to produce flat panels is plasma enhanced chemical vapor deposition (PECVD).
Plasma enhanced chemical vapor deposition is generally employed to deposit thin films on a glass substrate such as those utilized to fabricate flat panels. Plasma enhanced chemical vapor deposition is generally accomplished by introducing a precursor gas into a vacuum chamber that contains the substrate. The precursor gas is typically directed through a distribution plate situated near the top of the chamber. The precursor gas in the chamber is energized (e.g., excited) into a plasma by applying RF power to the chamber from one or more RF sources coupled to the chamber. The excited gas reacts to form a layer of material on a surface of a substrate that is positioned on a temperature controlled substrate support. The substrate support is typically grounded to the chamber body. In applications where a layer of low temperature polysilicon is deposited onto the substrate, the substrate support may be heated in excess of 400 degrees Celsius. Volatile by-products produced during the reaction are pumped from the chamber through an exhaust system.
During this process, the substrate support is biased negatively with respect to the plasma to further enhance deposition. This is accomplished by providing a bias voltage to an electrode within the substrate support assembly. With a negative bias voltage applied to the substrate support, positively ionized material in the plasma is attracted to and deposits on the substrate in a highly perpendicular manner, improving the deposition characteristic known as “step coverage”.
Ideally, the bias voltage on the substrate support remains stable as the ionized material is deposited onto the substrate. A stable bias voltage results in ionized deposition material being drawn and deposited uniformly across the width of the substrate. Voltage stability is realized when there is no appreciable voltage drop due to current flowing from the substrate support to ground. If the voltage drop is significant, the differential may induce plasma to strike between two points at substantially different voltages, such as the substrate support (at a high potential) and a nearby grounded feature (such as a chamber wall), thereby damaging the processing environment and possibly contaminating the substrate. Some systems employ a low impedance strap to couple the substrate support to the chamber body to facilitate grounding of the substrate support.
FIG. 10 is a simplified perspective, cutaway view of an exemplary conventional processing chamber 30 having a plurality of straps 20 for electrically coupling a substrate support 40 to a wall 32 or bottom 34 of the chamber 30. Four of eight straps 20 are shown in FIG. 10, two straps 20 coupled to each edge of the substrate support 40.
The substrate support 40 typically includes a plurality of lift pins 52, some of which are disposed along the edge of the substrate support 40 to lift the edges of the substrate during transfer. A lift plate 50 is disposed below the substrate support 40 and may be vertically actuated to extend the lift pins 52 through the substrate support 40 to space a substrate from the substrate support during substrate transfer.
Each of the straps 20 includes a first and second flexures 22, 24 separated by a bend 26. The straps 20 are generally aligned with the perimeter of the substrate support 40 and spaced to provide room for the lift pins 52 to extend below the substrate support 40. In order to provide clearance of a lift plate 50 positioned below the substrate support 40 that is utilized to vertically actuate the lift pins 52, the bend 22 of each strap 20 is oriented perpendicular to the proximate edge of the substrate support 40 (i.e., the edge of the support the strap is coupled to) to keep the bend 26 of the strap 20 from being positioned further inward relative to the substrate support 40 than the flexures 22, 24. As the straps 20 cannot extend into the area occupied by the lift plate 50 and lift pins 52, the number and size of the straps 20 are limited to the number that may be linearly aligned and nested along the edge of the substrate support 40, while remaining clear of those lift pins 52 positioned along the edge of the substrate support 40.
While this configuration has proven to be effective and reliable for smaller scale applications, it is less effective for larger area glass substrate processing applications which necessitate higher current flow for adequate grounding. As the next generation of large area substrates utilized for flat panel fabrication approaches 1100 mm×1300 mm, with even larger sizes envisioned for the near future, the substrate supports utilized to process these large area substrates have increased in size as well and would benefit from increased grounding capacity not currently available from conventional designs. The conductive straps such as those described above cannot be coupled between the processing chamber and the substrate support with sufficient density to adequately limit the voltage drop between the processing chamber and substrate support in such large scale processing applications. Additionally, because the straps are spaced around the perimeter of the substrate support to leave gaps for the lift pins and if the gaps are sufficiently wide, those portions of the substrate support between the straps may be biased at a higher potential relative to those portions that are directly coupled to a strap, which may adversely effect deposition uniformity.
Therefore, there is a need for a reliable low-impedance RF current return path suitable for use in large area substrate processing applications.